Fluorescent lamps of the type which use A.C. line operated ballast transformer auxiliaries are widely used in commercial and institutional buildings for illumination purposes. These buildings are generally overlit to insure that adequate light will be present for the worst case set of conditions, i.e., for night-time use with lumen depreciated, i.e., worn out, lamps or by a person having well below average visual acuity doing tasks requiring high lighting levels. Such overlighting can, of course, be reduced after it is determined what specific light levels are required for the tasks to be performed after a building is occupied. However, when standard ballasts and lamps are installed on a fixed distance ceiling grid, it is not always possible to reduce the lighting levels to those which meet minimum requirements and are also economical. For example, suppose a hallway has nominally 80 footcandles of conventional fluorescent lighting and it is later found that 10 to 20 footcandles would be adequate. The lighting level could be reduced approximately 15% with lower wattage "energy saving" lamps, i.e., standard 34 watt lamps, or by removing some of the lamps to reduce light levels. The latter unfortunately produces what is sometimes called "peak-and-valley" lighting. This type of lighting presents safety problems if for no other reason than the distance between the "on" lamps is increased so a light-dark or "peak-and-valley" lighting pattern is established.
The preferred approach is to keep all lamps "on" but, at a reduced power, thus reducing the light output level. The prior art includes a number of devices which permit some or all lamps to be operated at reduced power and light output levels. These devices are, however, generally limited to reducing the energy consumed to a maximum of 50% with a similar reduction in light output.
Manufacturers of this type of so-called "1/3" or "1/2" (33% and 50% reduction) "power reducers" usually accomplish this reduction by placing a capacitor (whose value determines amount of reduction) in series with one of the ballast transformer secondary leads and one of the lamp electrodes. However, if the lamp is of the rapid start fluorescent lamp type, such a connection is not possible due to the fact that the lamp electrode requires a two-wire path from a low voltage transformer winding which is close coupled to the ballast transformer primary. In particular, the connection in the secondary circuit of the ballast transformer cannot be because the case of the device is sealed, and to overcome this problem, prior art manufacturers add an external nominal 1:1 isolation transformer in series combination with the capacitor. This approach is more fully described in U.S. Pat. No. 3,954,316 (Luchetta et al). The need to add the isolation transformer also complicates the installation in that two of the ballast transformer secondary circuit wires (those going to the lamp) must be cut, and the insulation removed from the four ends, so the power reducer device can be connected. In addition, if the capacitor value is such that the current is limited to reduce energy consumption more than a nominal 50% of the rated energy consumption, the lamp will not ignite. Therefore, this type of secondary-installed "power reducer" is limited to capacitor values whose current limiting contribution does not inhibit lamp firing, i.e., those providing a nominal 50% reduction.
Other patents of possible interest include: U.S. Pat. Nos. 2,695,375 (Mendenhall et al); 3,235,769 (Wattenbach); 3,836,816 (Heck); 4,185,233 (Riesland); 4,207,497 (Capewell et al); 4,275,337 (Knoble et al); 4,399,391 (Hammer et al); and 4,496,880 (Luck).